A radioactive tracer, also called a radioactive label, is a substance containing a radioisotope that is used to measure the speed of chemical processes and to track the movement of a substance through a natural system such as a cell or tissue. A number of different radioactive forms of hydrogen, carbon, phosphorous, sulfur and iodine are commonly used in applications including biochemical assays, metabolism studies, and medical diagnostics.
A number of different radioisotopes are used as radioactive tracers depending on the application.
Tritium (3H) is a radioactive form of hydrogen that contains one proton and two neutrons in its nucleus. Tritium decays into helium-3 by emission of a low energy beta particle. The low energy of the emitted particle causes tritium to have low detection efficiency by scintillation counting. However, due to the abundance of hydrogen in organic compounds, tritium is frequently used as a tracer in biochemical studies.
Carbon-11 is a radioactive form of carbon that contains five neutrons and six protons in its nucleus. Carbon-11 decays into boron-11 by positron emission. Carbon-11 is frequently used as a tracer in positron emission tomography, an imaging technique that allows for three-dimensional imaging of functional processes in the human body.
Carbon-14 is a radioactive form of carbon that contains eight neutrons and six protons in its nucleus. Carbon-14 decays into nitrogen-14 by emission of a beta particle. Carbon-14 has been used as a tracer in both medical and scientific tests. Carbon-14 is frequently used to trace carbons through metabolic pathways.
Phosphorus-32 is a radioactive form of phosphorus that contains 17 neutrons and 15 protons in its nucleus. Phosphorus-32 decays into sulfur-32 by emission of a beta particle. Phosphorus-32 is frequently used to label amino acids and phosphoproteins and is commonly used to study protein phosphorylation by kinases in biochemistry. Phosphorus-32 emits a relatively high energy beta particle, and a number of safety precautions are needed when working with it.
Phosphorus-33 is a radioactive form of phosphorus that contains 18 neutrons and 15 protons in its nucleus. Phosphorus-33 decays into sulfur-33 by emission of a beta particle. Phosphorus-33 is used similarly to phosphorus-32, except that it emits less energetic beta particles, allowing for higher resolution assays and requiring less usage of safety equipment. Phosphorus-33 is less common and more expensive to produce than phosphorus-32.
Sulfur-35 is a radioactive form of sulfur that contains 18 neutrons and 16 protons in its nucleus. Sulfur-35 decays into chlorine-35 by emission of a beta particle. Sulfur-35 is frequently used as a tracer in biochemical experiments, where it is used to label amino acids and nucleic acids containing sulfur. Alternatively, a labeled sulfur can replace an oxygen in a phosphate group on a given nucleotide to create a thiophosphate which has very similar biochemical properties to the original phosphate group.
Iodine-123 is a radioactive form of iodine that contains 70 neutrons and 53 protons in its nucleus. Iodine-123 decays into tellurium-123 by electron capture, producing gamma rays. Iodine-123 is used in nuclear medicine imaging, specifically to study thyroid function. Because of its short half life, Iodine-123 is the most frequently used isotope in thyroid function studies.
Iodine-125 is a radioactive form of iodine that contains 72 neutrons and 53 protons in its nucleus. Iodine-125 decays into tellurium-125 by electron capture, producing gamma rays. Iodine-125 is frequently used in radio immunoassays because of its relatively long half life and ability to be detected with high sensitivity by gamma counters.